In the wake of two lacklustre debates which have raised a host of doubts about George W Bush's "IQ question", the front-runner in the race for the Republican presidential nomination has fallen behind John McCain in the key primary race in New Hampshire.

With one television chat-show host now labelling him a "bonehead", Mr Bush has endured the worst week of his campaign so far. A rash of newspaper columns have mocked the Texas governor for his rote speeches and a lack of gravitas, while Senator McCain continues to enjoy a honeymoon with the media.

Confirmation that Mr McCain has grabbed the lead in the first primary contest, where voting takes place on February 1, came in three opinion polls in New Hampshire this week. All the polls were taken after the first debate among Republican candidates, on December 2. The candidates had their second debate in Arizona, Mr McCain's home state, last Monday.

A Zogby/Reuters poll had Mr McCain leading by 35% to Mr Bush's 32%. A second poll, by the American Research Group, put Mr McCain in the lead by 37% to 30%. A third, by Franklin Pierce College, showed the biggest lead of all, with Mr McCain on 43% and Mr Bush on 28%.

The McCain surge came after Mr Bush, who had seemed invincible in his progress towards the Republican nomination for most of this year, gave debate performances that were initially judged to be adequate, but which are now being widely portrayed as unimpressive.

Last month Mr Bush had appeared to survive the embarrassment of being unable instantly to name the leaders of four developing nations when asked to do so by an interviewer. But the perception that has re-emerged from the debates has been that he is more of an intellectual lightweight than his crisply packaged early campaign performances had let on.

Two things in particular have contributed to this slippage. The first has been his deliberate strategy of not giving direct answers to questions and instead delivering chunks of his well-worn campaign speeches.

The second was triggered by a deceptively innocent question by Fox TV moderator Brit Hume in the December 2 debate. He asked what books Mr Bush had been reading.

Mr Bush, looking "like a possum cornered in the garage", according to the New York Times, replied that he "read books all the time" and was currently reading a biography of the post-war US secretary of state Dean Acheson.

In the second debate last Monday, Mr Bush was asked what lessons he had learned about the Acheson era from his reading. He answered very imprecisely, again drawing heavily on a line from one of his stump speeches, saying that "the lessons of Acheson and Marshall are - is that our nation's greatest export to the world has been, is, and always will be the incredible freedoms we understand in the great land called America".

He "looked like a ninth grader who has forgotten to do his homework, desperately free associating and hoping the bell would ring," the columnist Gail Collins wrote.

"The IQ question", as the Washington Post dubbed it this week, is now haunting Mr Bush. The TV chat-show host David Letterman calls him "a bonehead". Rival chat-show host Jay Leno says Mr Bush's slogan is "He'll get tough with, er, what's-his-name". Frank Bruni in the New York Times commented this week that "the most popular evolving caricature of Mr Bush "is the one perpetrated by Garry Trudeau in his Doonesbury strip: 'Nice suit; nobody in it'."

Mr Bush faced the question head-on with his characteristic good nature in Bedford this week. "They're saying I'm not very smart," he said. "I'd rather be under-estimated than overestimated. I've been under- estimated before. Governor Richards [whom Mr Bush ousted as Texas governor in 1994] regrets it."